Preheat oven to 400 degrees and line a baking sheet with parchment paper. Place a large skillet over medium heat and melt the butter. Add the onion, celery salt and pepper and cook for about 4 minutes until the veggies begin to soften. Add garlic and cook an additional 30 seconds.

Sprinkle flour over the veggies and stir to dissolve the flour. Allow the mixture to cook for about 30 seconds before adding the chicken broth. Stir to dissolve the flour in the chicken broth and bring the mixture to a bubble. Cook the filling on medium heat for about 3 minutes stirring constantly until nicely thickened.

Remove from heat and stir in cream or milk. Add the frozen vegetables and stir to incorporate. Once the veggies have been mixed in, fold in the chicken and taste for seasoning. At this point, the filling mixture should be at least room temperature if not slightly cool. If the mixture is still even slightly warm, place it in the refrigerator to cool while you roll out the pastry. It is imperative that the filling be cool before filling the pastry!

On a lightly-floured surface, roll the puff pastry into a rectangle about 14" wide by 10" deep. With the wide end toward you, spread the filling evenly over the dough, leaving a 1" border along the sides and a 2" border on top and bottom. Sprinkle the filling with shredded cheese, brush the edges with egg wash and roll the dough into a cylinder. Transfer the roll to the prepared baking dish with the seam on the bottom.

Press the sides of the roll together and tuck them under the cylinder. Brush the exterior with the remaining egg wash and bake for 30–35 minutes until deep golden brown all the way around. Allow the stromboli to cool for 10 minutes before slicing.